That thing is really cool!
What I think I'd do if it were mine:
1. Move the rear wheel mounts to behind the seat upright, stretching the wheelbase
2. Ditch the jackshaft and belt set-up
3. Flip the rear wheel and attach a standard chain sprocket
4. Geography allowing, find a cheap old snowblower with a Tecumseh 4-5hp flathead engine (it deserves the vintage look)
5. Get a band style brake assembly that grabs on the clutch hub
Whatever way you decide to go with it, it would be great to see the finished product.